Many of prior art aluminum electrolytic capacitors generally have the following structure. As shown in FIG. 1, a capacitor includes a capacitor element 2 having an anode 14 including aluminum anode lead tabs 12 and an aluminum anode sheet 11 connected together. The anode sheet 11 is formed of aluminum foil. Each anode lead tab 12 has a smooth surface, and the aluminum sheet 11 has its surface roughened and has an anodized film formed thereon. Each aluminum anode lead tab 12 is connected to the aluminum anode sheet 11 by welding at suitable locations 13. Alternatively, the tabs 12 and the anode sheet 11 may be connected together by needling the stack of the tabs 12 and the anode sheet 11, bending and pressing resulting burs down against the opposite surface of the stack. (This technique is referred to as "needling" hereinafter.) Further, the capacitor element 2 has a cathode 24 including aluminum cathode lead tabs 22 and an aluminum cathode sheet 21 connected together. The cathode sheet 21 is formed of aluminum foil. Each tab 22 has a smooth surface, and the aluminum cathode sheet 21 has its surface roughened. Each aluminum cathode lead tab 22 is connected to the aluminum sheet 21 by "needling" or welding at suitable locations 23. The anode 14 and the cathode 24 are rolled together into a cylindrical shape, with separator sheets 1 sandwiching either the anode 14 or the cathode 24. The roll is then impregnated with an electrolytic solution to thereby form the capacitor element 2.
Then, as shown in FIG. 2, the capacitor element 2 is encapsulated in a cylindrical aluminum casing 3. The casing 3 has an opening, which is hermetically sealed with a sealing structure 6 including a plate 4 of synthetic resin and a rubber plate 5 disposed on the resin plate 4. The sealing structure 6 has an explosion preventing valve 7 including an aperture formed through the resin sheet 4 and a thinned portion formed in the rubber sheet 5. Metallic terminal members 10 and 20 extend through the sealing structure 6. The anode lead tabs 12 extending out of the capacitor element 2 are joined together and connected to the terminal member 10 within the casing 3, and the cathode lead tabs 22 are joined together and connected to the terminal member 20 within the casing 3. The lower portion of the capacitor element 2 is fixed to the inner surface of the casing 3 with a fixing material 8.
Referring to FIG. 6(a), the anode 14 of the above-described prior art capacitor includes the aluminum anode sheet 11 with relatively thick anodic oxide films 31 on its opposite surfaces, which result from electrolytic processing at a high voltage above a capacitor rating voltage applied thereto. The aluminum cathode sheet 21 of the cathode 24 has thin oxide films 32 on its opposite surfaces, which result from spontaneous oxidation of the sheet 21 or from electrolysis with a low voltage of several volts. The aluminum sheets for the anode lead tabs 12 and the cathode lead tabs 22 has a thickness of about 200 .mu.m. The surfaces of the anode and cathode lead tabs 12 and 22 are not roughened by, e.g. etching. The surfaces of each anode lead tab 12 is covered with an electrochemically formed oxide film, while the surfaces of each cathode lead tab 22 is covered with an oxide film formed through spontaneous oxidation.
The above-described electrolytic capacitor charges and discharges in the following manner. As shown in FIG. 6(a), the electrostatic capacitance of the electrolytic capacitor can be considered to be a series combination of the capacitance exhibited between the anode sheet 11 and the separator 1 impregnated with the electrolytic solution, with the oxide film 31 interposed therebetween, and the capacitance exhibited between the aluminum cathode sheet 21 and the separator 1 with the oxide film 32 interposed therebetween. Since the oxide film 32 is considerably thin relative to the oxide film 31, the capacitance associated with the oxide film 32 should be far larger than the capacitance associated with the oxide film 31. On the other hand, a very large leakage current is associated with the oxide film 32. Accordingly, when a voltage V is applied between the anode sheet 11 and the cathode sheet 21 as shown in FIG. 6(a), the voltage Va across the oxide film 31 is larger than the voltage Vc across the oxide film 32. The apparent capacitance per unit area of the capacitance associated with the oxide film 31 is Ca (.mu.F/cm.sup.2), and the apparent capacitance per unit area of the capacitance associated with the oxide film 32 is Cc (.mu.F/cm.sup.2). The amounts of charge stored in these capacitances are Qa and Qc, respectively.
When the two terminals of the above-described electrolytic capacitor charged to the voltage V are connected together, the two capacitances Ca and Cc are connected in parallel as shown in FIG. 6(b), so that the voltage between the two terminals becomes Vc' due to dicharge of the charge on the smaller capacitance Cc, and charge of Qa-Qc remains. Since the overall capacitance is Ca+Cc and the stored charge is Qa-Qc, the remaining voltage Vc' is expressed by the following expression (1). ##EQU1##
If the voltage applied across the cathode oxide film 32 during discharging is excessive, an oxide film may be further grown on the cathode sheet 21, which may cause undesirable things, such as generation of gas, to occur within the capacitor casing 3. Then, the remaining voltage Vc' expressed by the expression (1) must be equal to or smaller than the maximum voltage V' which can be applied across the cathode oxide film 32 without growing any additional oxide film during discharging. In other words, the condition expressed by the following expression (2) must be met during discharge. ##EQU2##
Since Va=V-Vc, the following expression (3) can be derived from the expression (2). ##EQU3##
A ripple waveform resulting from rectifying an AC voltage and a rectangular charge and discharge voltage waveform contain portions in which the voltage rapidly changes from the maximum value to the minimum value in short time intervals. If the condition expressed by the expression (3) is met, no oxide film growth takes place on the cathode sheet 21 even when such rapidly changing current or voltage is applied to the cathode sheet 21. In prior art, a major attempt to improve the ripple insensitivity and the charge-and-discharge insensitivity of an electrolytic capacitor has been to fulfil the condition expressed by the expression (3). For example, it has been done to use a cathode sheet having a large capacitance per unit area, or a sheet with an additional oxide film intentionally pre-formed on it having a high withstand voltage. The term "ripple insensitivity" is used in this specification to represent a property of a sheet of, for example, aluminum, that an oxide film does not grow or hardly grow on the sheet when ripple current above an allowable magnitude is applied to the sheet. The term "charge-and-discharge insensitivity" used in this specification is a measure indicating how an oxide film does not grow when a large voltage difference occurs between charging and discharging of a capacitor.
There is a limit to the prior art improvement of the ripple insensitivity and the charge-and-discharge insensitivity of an electrolytic capacitor. The inventors have conducted experiments on electrolytic capacitors which have been judged to be sufficiently ripple and charge-and-discharge insensitive. They have made analysis of such electrolytic capacitors used in circuits in which ripple current having a magnitude larger than an allowable limit is applied, and electrolytic capacitors used in circuits in which the difference between the voltage to which the capacitors are charged and the voltage to which the capacitors are discharged is large. They have found that even in such capacitors employing ideal or approximately ideal cathode sheets 21, a film-forming reaction takes place on the cathode lead tabs 22 and adjacent portions of the cathode sheet 21, causing gas to be generated within the capacitor, which undesirably results in the opening of the explosion preventing valve 7 due to the rise of the pressure within the capacitor.
Therefore, an object of the present invention is to provide an electrolytic capacitor having improved charge-discharge insensitivity and ripple insensitivity against charging and discharging and against conduction of ripple current, by virtue of a structure which can prevent production of an oxide film on a cathode side during discharging of the capacitor.